The invention relates to cooling equipment, especially for the cryopreservation of biological samples.
The freezing of biological samples such as, e.g., stem cells in order to preserve their vitality is known within the scope of so-called cryopreservation. A cooling down to less than −130° C. is necessary here for a complete preservation of vitality so that liquid nitrogen is usually used as cooling agent. However, not only the low storage temperature is important for the preservation of vitality but also the observance of a given temperature course in time during freezing and thawing.
In order to meet these requirements, cooling equipment is obtainable, e.g., from DE 88 07 267.3 that uses liquid nitrogen with a boiling point of −196° C. as cooling agent. The liquid nitrogen is at first located in a cooling agent storage container and is heated in it by an electrically operated evaporator, the outgassing nitrogen being conducted via a cooling agent supply line into a cooling chamber and correspondingly cools its inner space so that material to be cooled located in the cooling chamber is frozen.
However, the mere outgassing of nitrogen by the evaporator only makes cooling agent temperatures near the boiling point of −196° C. possible, whereas, on the other hand, the cooling chamber should also be cooled to higher temperatures, especially during the freezing and thawing. Therefore, an electrically operated heater that heats the outgassing nitrogen to the desired temperature is arranged in the cooling agent supply line between the cooling agent storage container and the cooling chamber.
Furthermore, the known cooling equipment comprises a control device that measures the temperature of the cooling agent introduced into the cooling chamber as a control variable and adjusts the heating performance of the heater arranged in the cooling agent supply line as a manipulated variable in order to achieve the desired temperature course in time during freezing and thawing. Thus, the control device controls only a single heater and evaluates only a single temperature.
However, the previously described, known cooling equipment has the disadvantage of an unsatisfactory control behavior, which expresses itself in an overswinging between the target temperature and the actual temperature and results in a deviation from the desired temperature course in time during freezing and thawing. As a result, the unsatisfactory control behavior of the known cooling equipment can result in damage to the biological samples to be preserved.
The invention therefore has the task of improving the temperature control behavior in the previously described, known cooling equipment.